In rapid thermal processing (RTP), workpieces such as semiconductor wafers can be subjected to specified temperature cycles of arbitrary complexity. For this reason, RTP is useful for carrying out thermally dependent processes, such as diffusion and annealing, in the course of manufacturing integrated circuits. However, some of these processes require the temperature to be controlled within limits as small as .+-.10.degree. C. or less. Such fine control is possible only if the wafer temperature can be measured to precision that is comparably high. Although thermocouples installed directly on the wafers will give precise temperature measurements, the instrumented wafers will generally be unsuitable for forming into integrated circuits. If thermocouples are installed on only a few wafers in each batch, individual variations between wafers and between locations within the RTP reactor may cause unacceptable deviations between the actual and inferred temperatures of the non-instrumented wafers.
Optical pyrometry is a useful alternative to direct instrumentation of the wafers within the reactor. One pyrometric technique has been described in U.S. Pat. No. 5,154,512, issued to C. W. Schietinger et al. on Oct. 13, 1992. This technique is schematically illustrated in FIG. 1. According to this technique, a first light-pipe probe 10 is provided, having an input aperture that faces wafer 20, and a second probe 30 is provided having an aperture that faces one of opposing lamp banks 40, which are typically linear arrays of quartz-tungsten-iodine lamps situated outside of processing chamber 50. First probe 10 samples radiation emitted and reflected by the wafer and directs the sampled radiation into detector 60. Second probe 30 samples radiation emitted by the lamps and directs the sampled radiation into the detector. Probe 30 receives radiation both in a direct path from the lamps and also by reflection from reflector 80. The emissivity .epsilon. of the wafer is inferred from the probe signals, and then the wafer temperature is inferred from the Planck radiation law, which relates the wafer thermal emittance w, the wafer emissivity .epsilon., and the wafer temperature T.
As noted, the first probe signal is a sum of emitted and reflected radiation. Information sufficient to resolve the emitted and reflected components is available because the emission from the lamps, which are driven by alternating current, has an ac component, referred to as "ripple." Because the thermal emission from the wafer has no significant ac component, the wafer reflectivity is estimated as the ratio of the ripple amplitudes in the first and second probe signals, respectively. After this reflectivity has been evaluated, the first probe signal is corrected to yield a resolved value of the wafer thermal emittance.
Although useful, this technique fails to take into account several potentially significant sources of systematic error. One such error involves the determination of wafer reflectivity. The theoretical temperature computation implicitly assumes that this reflectivity is the hemispherical reflectivity of the wafer. However, the reflectivity actually measured can diverge significantly from the hemispherical reflectivity, because both probes sample the radiation field from a relatively small area.
That is, an excellent approximation to hemispherical reflectivity could be provided if: probe 10 were to sample wafer reflections from a relatively small area of the wafer, probe 30 were to sample the lamp radiation over a hemispherical volume (i.e., over a solid angle approaching 2.pi. steradians), and local fluctuations in lamp output and wafer reflectivity could be neglected. The second of these conditions is not fully satisfied, because most of the radiation collected by probe 30 enters the probe through end surface 90, which is typically a flat surface. As a consequence, probe 30 will typically collect radiation only over a view angle of 90.degree. or less. This will generally lead to a significant divergence between the measured reflectivity and the hemispherical reflectivity.
Another potential systematic error involves local fluctuations in lamp output. Because, as noted, the second probe samples only a relatively small area, it may be ineffective for averaging these fluctuations.
Thus, practitioners in the field have hitherto failed to provide a pyrometric technique so robust against potential sources of systematic error that temperature measurements of .+-.10.degree. C. accuracy or better can be made routinely at typical processing temperatures.